In the chemical industry, heat-recovery boilers are widely employed to exploit the waste heat from upstream processes for steam generation, in that hot process gases, typically having a temperature from 800 to 1300° C., are cooled and simultaneously high-pressure steam is generated. In this case, deposits may form on the inner surface of the pipes, which significantly impair the heat transfer between the process gas and the coolant liquid flowing around the pipes because of their comparatively low thermal conductivity. The formation of such deposits is to be attributed either to materials existing in the process gases or to those materials which first form in the pipes upon cooling of the process gases. The process of the occurrence of such disadvantageous deposits is referred to in the professional world as “fouling.” In order to limit fouling in the pipes, providing the process gases with a sufficient flow velocity is known. However, since the flow velocity may not be elevated unlimitedly for reasons of a pressure loss which rises therewith, displacement bodies are inserted into the cooling pipes in a known way, which serve to either elevate the turbulence of the process gas flow or even elevate its flow velocity locally, as a result of which the deposits of solids are reduced and simultaneously heat transfer is improved. The displacement bodies, which typically are made of metal and are preferably formed as closed insertion tubes, are disadvantageously subject, particularly if the process gases have high concentrations of carbon monoxide, to a significant corrosive attack, referred to as “metal dusting,” in a temperature range from 400 to 850° C., preferably 450 to 750° C. Metal dusting is based on the enrichment of the matrix of a metallic material in the surface region with carbon, carbide compounds first arising and, upon further saturation, elementary carbon being precipitated. The material structure is destroyed by the precipitation of the carbon, so that it erodes. A requirement for such erosion is the presence of a potential for carbon formation. This potential may be defined by the following reaction equations in the components of a gas mixture obtained through gasification of carbon:CO+H2C+H2O (CO reaction)2C0C+CO2 (Boudouard reaction).
The associated equilibrium temperature may be determined for each of the two reactions from the composition of the gas generated by the gasification. Since both reactions run exothermically, a potential for producing carbon exists if the gas falls below at least one of these temperatures as it is cooled. Whether metal dusting actually occurs is decisively a function of the associated kinetics, whose influencing variables are determined by the local temperature and the material. The temperature limit, below which metal dusting no longer occurs for reasons of kinetics, may be viewed as relatively well-established on the basis of experience, however, it is still largely open how suitable different metallic materials are for use above this temperature limit. In principle, all iron and nickel alloys are susceptible to metal dusting, however, a more or less strong occurrence of metal dusting occurs as a function of the further components determining the mechanical-technological properties of alloys. Up to this point, the development of a material resistant to metal dusting has not been successful, nor is there a sufficiently established theory about the detailed procedures during metal dusting.
In general, metal dusting may be avoided if the displacement bodies are only subjected to process gases whose temperatures lie above or below the critical temperature range of 400 to 850° C. In the heat-recovery boiler, the pipes that the hot process gases flow through are cooled to temperatures lying significantly below 400° C. by the coolant liquid surrounding them and the vaporization reaction occurring. Since, however, the use of displacement bodies in the critical temperature range of 400 to 850° C. may not be avoided under all circumstances, it is necessary to consider the risk of metal dusting when selecting the material for the displacement body.